Correct or improve the translation of the following Chinese sent

Correct or improve the translation of the following Chinese sentences.原文：理论上看起来容易的事情做起来却往往非常困难。 译文：That seems easy in theory is often immensely difficult to achieve.改译：
【正确答案】：

WHAT SEEMS EASY IN THEORY IS OFTEN IMMENSELY DIFFICULT TO ACHIEVE.

【题目解析】：“理论上看起来容易的事情”在句中作主语，又是从句，并在从句中作主语，因此引导主语从句应用引导词what。